CODEWITHSUNDEEP
ocaml
齐天大圣
齐天大圣

Reputation: 1189

Printing promt before user input

I am new to OCaml, and now I am trying to make a simple REPL.

let rec repl () = 
    print_prompt () ;
    let input = Scanf.scanf "%s" (fun x -> x) in
    if input = "" then repl ()
    else print_endline input

let print_prompt () = print_string "> "

The problem now i am having is: when program starts, it does not display prompt immediately. It waits for my input and prints prompt along with my input.

What I want is:

> "user_input"
"user_input"

But i am getting :

"user_input"
> "user_input"

How can I fix this?

Upvotes: 1

Views: 565

Answers (3)

Pierre G.
Pierre G.

Reputation: 4425

Using readline instead of Scanf :

val read_line : unit -> string

Flush standard output, then read characters from standard input until a newline character is encountered. Return the string of all characters read, without the newline character at the end.

Upvotes: 4

Jeffrey Scofield
Jeffrey Scofield

Reputation: 66803

This is almost certainly a buffering problem. In your print_prompt function, flush the standard output:

flush stdout

Upvotes: 2

ivg
ivg

Reputation: 35210

Well, you didn't show the print_promt implementation, but I can guess, that it uses some buffered io function like print_string or printf. They print into an intermediate buffer and data will not be displayed unless flush is called. You can use flush or flush_all functions to do this manually. Also you can use a special specificator %! in printf formats string:

 open Printf

 let print_prompt () = printf "> %!"

Upvotes: 2

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